Add documentID()

Author: schmidt-sebastian

packages/firestore/lite/index.node.ts

@@ -39,6 +39,9 @@ export {
 
 export { FieldPath } from './src/api/field_path';
 
+// TOOD(firestorelite): Add tests when Queries are usable
+export { documentId } from './src/api/field_path';
+
 export { DocumentSnapshot, QueryDocumentSnapshot } from './src/api/snapshot';
 
 export function registerFirestore(): void {

packages/firestore/lite/src/api/field_path.ts

@@ -18,7 +18,10 @@
 import * as firestore from '../../index';
 
 import { tryCast } from './util';
-import { FieldPath as InternalFieldPath } from '../../../src/model/path';
+import {
+  DOCUMENT_KEY_NAME,
+  FieldPath as InternalFieldPath
+} from '../../../src/model/path';
 import { validateNamedArrayAtLeastNumberOfElements } from '../../../src/util/input_validation';
 import { Code, FirestoreError } from '../../../src/util/error';
 
@@ -69,3 +72,7 @@ export class FieldPath implements firestore.FieldPath {
     return this._internalPath.isEqual(path._internalPath);
   }
 }
+
+export function documentId(): firestore.FieldPath {
+  return new FieldPath(DOCUMENT_KEY_NAME);
+}